Interpretive Summary: Because of their small size and infrequent feeding, it is difficult to tell whether predators are consuming significant numbers of insect pests. The polymerase chain reaction can detect pest DNA in the guts of insect predators, but because different predator species digest prey remains at different rates, merely knowing the proportion of predators containing prey remains won’t give an accurate measure of their relative impact on a pest. To solve this problem, we invented the detectability half-life to compensate for differences in digestive rate. In a field study of predators of the Colorado potato beetle (CPB), we found that two stink bug species had the highest incidence of CPB DNA in the gut. But when the gut assays of the collected predators are corrected for these differences, another species, a ground beetle that had next-to-lowest CPB DNA incidence in the gut, is ranked next-to-highest, making it a better target for conservation than one of the stink bugs. This information is important to entomologists and pest managers designing farming systems to conserve natural enemies of CPB. The approach is applicable to other predator-pest systems as well, and is therefore a general advance in biological control technology.

Technical Abstract:
Agroecosystems are speciose, making selection of natural enemies for conservation biological control non-trivial. Molecular gut analysis enables ranking of predators by the incidence of pest remains in the gut. However, predators differ in digestive rates, and ranking by incidence favors those with slower rates. Multiplying the incidence of the pest in each predator species by the ratio of its half-life to that of the longest half-life in the predator complex rectifies this disparity. The Colorado potato beetle (CPB) predator complex includes four species in two insect orders and three families. These species and their stages display order-of-magnitude variation in half-life for the Cytochrome Oxidase I sequence of a single CPB egg, from 7.0 h in larval Coleomegilla maculata to 84.4 h in nymphal Perillus bioculatus. The incidence of CPB DNA in the guts of 351 predators collected from conventionally tilled fields ranged from 11% in C. maculata adults to 95% in P. bioculatus nymphs, ranking them thusly: C. maculata adults < Lebia grandis adults < P. bioculatus nymphs < P. bioculatus adults < Podisus maculiventris nymphs < P. maculiventris adults, making P. maculiventris the best candidate for conservation and L.grandis and C. maculata the worst. The half-life correction changes the rankings: C. maculata adults < P. bioculatus nymphs < P.maculiventris nymphs < P. bioculatus adults < L. grandis adults < P. maculiventris adults. These changes, especially the elevation of L. grandis from next-to-worst to next-to-best candidate, demonstrate the importance of half-life-weighting for targeting candidate predators in conservation biological control programmes.